TMP17

REV. 0

–4–

THEORY OF OPERATION

The TMP17 uses a fundamental property of silicon transistors

to realize its temperature proportional output. If two identical

transistors are operated at a constant ratio of collector current

densities, r, then the difference in base-emitter voltages will be

(kT/q)(ln r). Since both k, Boltzmann’s constant, and q, the

charge of an electron, are constant, the resulting voltage is

directly Proportional To Absolute Temperature (PTAT). In the

TMP17 this difference voltage is converted to a PTAT current

by low temperature coefficient thin film resistors. This PTAT

current is then used to force the total output current to be

proportional to degrees Kelvin. The result is a current source

with an output equal to a scale factor times the temperature (K)

of the sensor. A typical V-I plot of the circuit at 125

°C and the

temperature extremes is shown in Figure 6.

Factory trimming of the scale factor to 1

µA/K is accomplished

at the wafer level by adjusting the TMP17’s temperature

reading so it corresponds to the actual temperature. During

laser trimming the IC is at a temperature within a few degrees of

25

°C and is powered by a 5 V supply. The device is then

packaged and automatically temperature tested to specification.

FACTORS AFFECTING TMP17 SYSTEM PRECISION

The accuracy limits given on the Specifications page for the

TMP17 make it easy to apply in a variety of diverse applica-

tions. To calculate a total error budget in a given system it is

important to correctly interpret the accuracy specifications, non-

linearity errors, the response of the circuit to supply voltage

variations and the effect of the surrounding thermal environ-

ment. As with other electronic designs external component

selection will have a major effect on accuracy.

CALIBRATION ERROR, ABSOLUTE ACCURACY AND

NONLINEARITY SPECIFICATIONS

Two primary limits of error are given for the TMP17 such that

the correct grade for any given application can easily be chosen

for the overall level of accuracy required. They are the calibra-

tion accuracy at

25

°C, and the error over temperature from

40

°C to 105°C. These specifications correspond to the

actual error the user would see if the current output of a

TMP17 were converted to a voltage with a precision resistor.

Note that the maximum error at room temperature or over an

extended range, including the boiling point of water, can be

directly read from the specifications table. The error limits are a

combination of initial error, scale factor variation and non-

linearity deviation from the ideal 1

µA/K output. Figure 2

graphically depicts the guaranteed limits of accuracy for a

TMP17GS.

The TMP17 has a highly linear output in comparison to older

technology sensors (i.e., thermistors, RTDs and thermo-

couples), thus a nonlinearity error specification is separated

from the absolute accuracy given over temperature. As a

maximum deviation from a best-fit straight line this specification

represents the only error that cannot be trimmed out. Figure 8

is a plot of typical TMP17 nonlinearity over the full rated

temperature range.

0.2

0.1

0

0.1

0.2

TEMPERATURE – C

40

105

25

0

25

70

TYPICAL NONLINEARITY

Figure 8. Nonlinearity Error (TMP17)

TRIMMING FOR HIGHER ACCURACY

Calibration error at

25

°C can be removed with a single

temperature trim. Figure 9 shows how to adjust the TMP17’s

scale factor in the basic voltage output circuit.

+V

TMP17

R

100

Ω

950

Ω

Figure 9. Basic Voltage Output (Single Temperature Trim)

To trim the circuit the temperature must be measured by a

reference sensor and the value of R should be adjusted so the

output (V

procedure should be implemented as close as possible to the

temperature highest accuracy is desired for. In most applications

if a single temperature trim is desired it can be implemented

where the TMP17 current-to-output voltage conversion takes

place (e.g., output resistor, offset to an op amp). Figure 10

illustrates the effect on total error when using this technique.

AFTER SINGLE

TEMPERATURE

CALIBRATION

ACCURACY

WITHOUT TRIM

1.0

0.5

25

105

25

0

0.5

1.0

TEMPERATURE – C

40

Figure 10. Effect of Scale Factor Trim on Accuracy